The present invention relates to an articulated transmission joint with quadruple drive link.
FR-A-2,566,858 discloses such a transmission joint comprising a cross-pin whose four radial arms, connected to one of the sides of the transmission, are engaged in four longitudinal tracks connected to the other side of the transmission.
Such joints are capable of transmitting very high speeds of rotation without significant friction or wear, and without any vibration inherent in the kinematics of the joint being transmitted to the input and output shafts. This is because, on the one hand the joint is homokinetic, and on the other hand it is capable of performing a complete revolution with strict immobility of its geometrical input and output axes, even when they form an angle between them. On the contrary, with an articulated joint having only a triple drive link (joint with tripod and three tracks), at least one of the axes performs a slight planetary movement in the course of a revolution, and this produces vibrations at very high speed.
In order for the four drive links to be capable of coexisting without mechanical tension in the joint, it is necessary for the tracks or the trunnions to be mounted elements which oscillate with respect to the side of the transmission with which they are associated.
In fact, during operation at an angle, when the plane of the axes of the two members of the joint forms an angle with respect to each of the trunnion axes, the centres of the trunnions would no longer be located in the axis of the tracks if it were not possible for the trunnions or the tracks to become slightly offset.
The offset necessary for each track or trunnion is opposed to that which is necessary for its two neighbours. The oscillating elements (tracks or trunnions) are coupled angularly to one another so as to impose this phase opposition between each oscillating element and its two neighbours. In the absence of such a coupling, all of the oscillating elements would roll over in the same direction under the action of the torque to be transmitted.
Thus, there are four oscillating elements and, at any point in time, their movements are equal and opposite, so that the forces of inertia which result from this cancel one another and do not produce any torsional vibration on either side of the transmission. Consequently, despite the oscillation of the oscillating elements, joints with quadruple drive link are very well suited not only to lateral transmission shafts ending at the wheels, but also to very high speeds of rotation, for example 6,000 rpm, which are encountered upstream of the differential particularly in the longitudinal transmission shafts in vehicles with rear wheel drive or with four wheel drive. Moreover, on these joints, the maximum break angles encountered in practice are very small, which limits the amplitude of the oscillations of the oscillating elements and therefore the amplitude of the to-and-fro movement which must be made along each trunnion arm by the spherical roller which is generally provided thereon. This movement is necessary in order for the roller to remain centred on the axis of the corresponding track throughout each revolution of the joint when the joint is operating at an angle.
The frequency of oscillation and of the oscillating elements is twice the speed of rotation of the joint. Thus, for a speed of rotation of 100 rps (6,000 rpm), for example, the frequency of oscillation of the oscillating elements is 200 Hertz. Although the amplitude of the oscillation movements is small--a few degrees--, the inertia forces brought into play are, at such frequencies, far from negligible.
U.S. Pat. No. 4,571,214 discloses a pseudo-spherical roller which is specially designed for high-speed homokinetic joints. This pseudo-spherical roller, slideably mounted on an arm of the cross-pin, can contact the edges of its two rolling paths, simultaneously and without damage, during relaxations of the torque or when the torque is reversed, even when the effect of the centrifugal force is greater than that of the torque to be transmitted.
Now, in the joint according to FR-A-2,566,858, it is the force of contact between the tracks and the arms of the cross-pin (that is to say, in the example, between the tracks and the rollers provided on the arms of the cross-pin) which ensures the reciprocating oscillation of the oscillating tracks--or trunnions--by communicating to them the reciprocating forces which are necessary for them to overcome their inertia. The reciprocating forces of inertia to be overcome can be of the same order of magnitude as the centrifugal force applied to the rollers. This centrifugal force is therefore not capable, by itself, of maintaining the roller bearing simultaneously against both rolling paths. This may result in losses of contact and in audible impacts when contact is restored between the rollers and their rolling paths, when the rollers bear alternately on one track and on the other in order to produce the reciprocating oscillation force. This is especially true when the torque transmitted is too weak to favour the bearing against one of the two rolling paths.